1. Field of the Invention
The present invention relates to a rear monitoring system for a vehicle, and more particularly to a rear monitoring system for detecting another vehicle approaching one's own vehicle during vehicle running using an image picked up by a vehicle-loaded camera and giving a warning to a driver. Incidentally, it should be noted that the rear includes a rear-side or a diagonal rear.
2. Description of the Related Art
As an example of such a rear monitoring system for a vehicle, a previously known technique is to detect another vehicle approaching one's own vehicle using optical flows from another vehicle. Now referring to FIGS. 9A-9D, an explanation will be given of a conventional rear monitoring system for a vehicle.
FIGS. 9A-9C are views for explaining a change in a rear background image acquired by a camera 1. FIG. 9A shows a status inclusive of one's own vehicle. FIG. 9B shows an image picked up by the camera 1 at timing t in an environment of one's own vehicle. FIG. 9C shows an image picked up at timing t+.DELTA.t.
Now it is assumed that one's own vehicle is running straight on a flat road. The road sign and building residing in the rear of one's own vehicle in FIG. 9A are observed as images shown in FIGS. 9B and 9C at timings t and t+.DELTA.t, respectively. Coupling the corresponding points in these two images provides speed vectors as shown in FIG. 10D. The are referred to as "optical flows". Where a following vehicle approaches, the directions of the vectors in the optical flows in FIG. 10D are contrary. Using the optical flows, the conventional environment monitoring system monitors the relationship between one's own vehicle and the following vehicle or another vehicle running on an adjacent lane to detect the other vehicle approaching one's own vehicle, thereby giving a warning to a driver.
Meanwhile, when the vehicle is backed up or reversed, there is a range out of a field of view of a driver even if the field of view in the rear is supplemented with a fender mirror or rear view mirror. Particularly, a large-scale vehicle such as a truck and bus has a large blind spot in a rear or rear-side direction from a vehicle driver. In order to obviate such an inconvenience, it was proposed to assure a rear-side field of view using a rear-side road image taken from a camera installed on the back of the large-scale bus and monitor the rear-side direction while the vehicle is reversed.
While the vehicle is reversed, the camera used to monitor the rear-side direction must pick up the range within several meters from the back in order to confirm an obstacle in the vicinity of the vehicle. On the other hand, while the vehicle is forwarded, the camera must pick up the place far from the back of the vehicle by several meters or more in order to detect optical flows generated from the vehicle which follows one's own vehicle or runs on an adjacent vehicle lane and approach from the long distance.
In this way, the camera used to monitor the rear direction while the vehicle runs forward and backward has different ranges to be picked up. Therefore, a single camera could not be used to monitor the rear-side direction both while the vehicle runs forward and backward. In order to obviate such an inconvenience, it was proposed to use a wide-angle lens capable of picking up the place near to and far from one's own vehicle. However, this proposal lowers the resolution of the entire road image so that detection of optical flows is made difficult.
A prior art of detecting optical flows detects the white lines of one's own lane on a straight road, defines, as an FOE point, the crossing point of extended lines of the detected white lines and makes correlation processing radially from the FOE to acquire optical flows, thereby realizing the image processing at a high speed. However, the road image picked up by the camera in the night time does not have high contrast so that it is difficult to detect the white lines. In order to obviate such an inconvenience, it can be proposed to detect the white lines in the vicinity of the vehicle by illuminating the rear of one's own vehicle with light to a degree that safe running of the following vehicle is not hindered, and to estimate the white lines on the road image.
However, since the camera used to monitor the rear-side direction while one's own vehicle runs forward picks up the far place, the single camera cannot be used for the rear-side monitoring and detection of the white lines in the vicinity of the vehicle while one's own vehicle runs forwards. In this case also, it can be proposed to use a wide-angle lens capable of picking up the place near to and far from one's own vehicle. However, this proposal lowers the resolution of the entire road image so that detection of optical flows is made difficult.